"I've been waiting for this day for 31 years," said Eugene A Cernan, the last astronaut to set foot on the Moon.

Russia and China also have ambitions for new space exploration but some argue that future trips are best left to robots like the rover now on Mars.

President Bush's "new vision" for American space exploration calls on the United States to:

send astronauts back to the Moon as early as 2015, no later than 2020

use human and robotic exploration of Moon to prepare for living base and missions to Mars

return the space shuttle to flight but retire it by 2010

develop a shuttle replacement by 2008 for manned exploration by 2014

finish US work on the International Space Station (ISS) by 2010

Speaking at the US space agency (Nasa) headquarters in Washington, Mr Bush said astronauts had recently been going no further from the Earth than 621 kilometres (386 miles).

"It is time for America to take the next steps... and extend a human presence across our Solar System," he said.

Experience gained from new journeys to the Moon would be used in "human missions to Mars and to worlds beyond", the president said.

"Human beings are headed into the cosmos."

'No space race'

Mr Bush added that he wanted a new era of discovery but not a new space race.

However, scientists in Russia, whose spacecraft currently service the ISS, announced on Wednesday that they had plans for manned flights to Mars within a decade.

"Technically, the first flight by earthlings to Mars could take place as early as 2014," said Leonid Gorshkov, a designer at the Russian space corporation Energiya.

Mr Gorshkov put the cost at about $15bn, saying that America planned to spend 10 time that amount on its Mars programme.

China plans to put an unmanned vehicle on the Moon by 2010.

Spreading the cost

The White House estimates that the Moon plans will require an extra $12bn over the next five years.

Mr Bush was introduced by orbiting astronaut Michael Foale via video

It said $11bn of that would come from reallocation of Nasa funds to the new priorities and the president would ask Congress to approve the remaining $1bn.

Mr Bush would ask for a 5% yearly increase in Nasa's $15.4bn annual budget for the next three years, followed by rises of up to 1% in subsequent years.

A total estimate for the cost of sending astronauts back to the Moon and on to Mars was not given - and will have to be approved by Mr Bush's successors in the White House.

Mr Bush highlighted the practical uses of space technology, such as image processing used in medical scanners, foetal heart monitors and insulin pumps.

The president announced the development of a new craft, the Crew Exploration Vehicle, presented as the first since the Apollo spaceships to take humans to new worlds.

It would also be able to transport scientists and crew to the ISS.

'Robots do it better'

The head of Nasa, Sean O'Keefe, predicted that the Moon plans would not be a "budget-buster".

However, critics say Mr Bush's return-to-space idea is irresponsible at a time when the federal budget deficit is soaring.

Democratic presidential candidate Joseph Lieberman said the US should not be "going hundreds of millions of miles away on a costly new mission when [it had] limited resources".

The president's father proposed sending men to Mars when he was in office in 1989, but the project went nowhere after its cost was estimated at up to $500bn.

The new announcement comes as the US space programme celebrates the landing of its robot rover Spirit on Mars and its planned exploration.

Professor Robert Park of the University of Maryland suggested that the best way forward for space exploration was through robots such as the rover, which were not subject to the physical limitations of human beings.

"The great adventure of our time... is to explore where no human could ever set foot," he told the BBC's World Today programme.